Originally Posted by
keith smith
2.I still maintain that cg can be significantly behind wing cp because one purpose of tailplane is to make wing+tail cp behind cg,thus ensuring static stability. In such a case, for trim,tail lift would have to be upwards.
However, if you wanted the wing cp to be behind the cg, then the trim load on the tail would be downwards (I hope I have got that the right way round---I am sure somebody will tell me if I havn't)
Keith
Sorry, not true in a couple of regards.
The purpose of the tailplane is not to locate the CP in any particular place; it is to make the aircraft stable (and controllable, but lets stick with the "Stability" part of S&C for now).
An aircraft is stable in pitch when the gradient of pitching moment coefficient with respect to lift coefficient is negative - which means that if I disturb the aircraft so as to create more lift (and more CL - which USUALLY means more AOA) then I also get a more negative pitching moment, which will pitch me back nose down again, opposing the initial disturbance.
Therefore what matters in stability are the so-called neutral points - the notional cg position where d(Cm)/d(CL) is zero. At those points the aircraft is neutrally stable in pitch - any further aft and its unstable. There are two neutral points one may consider - one tail-off and one tail-on (for a conventional layout, naturally). The tail-on one includes the stabilising effect of the tail, as one would expect.
For a naturally stable aircraft the cg range MUST be ahead of the tail-on neutral point; it may, and often is, behind the tail-off neutral point. Most aircraft would be unstable if the tail fell off, therefore.
Now, all that only relates indirectly to both the load on the tailplane and the cp, because the relationship between cp and pitching moment is not direct - a cambered airfoil will generate a pitching moment even at zero lift - mechanically, a 'couple' - so some tail load is required for trim even where there is no wing lift (and where the wing cp is basically undefinable). And because the tailplane lift is a trim force, it depends on the actual magnitude of the pitching moment - whereas stability is concerned with the gradient of the pitching moment.
So while the cg can indeed be behind the wing cp, that says nothing special about the tail lift (and, indeed, since most airfoils will generate a nose-down pitching moment, it's likely that you'd need download on the tail, even if the cg were at or behind the wing cp. There ARE indeed circumstances where one can have an uploaded tail on a stable aircraft, but they are rather less common than your post suggests, and don't apply to all aircraft. There's been some discussion on this last point a number of times IIRC.